The development of epitaxial oxide based spin polarized devices has recently focused on so-called colossal magnetoresistance (CMR) materials (or doped perovskite manganites). The details of the magnetic response in such a device have been strongly affected by the complex magnetic domain state. Such micromagnetic effects have greatly complicated the interpretation of transport data and may contribute to the premature suppression of large magnetoresistance to temperatures well below the Curie temperature of the CMR electrodes. We have observed the local magnetic structure of submicron CMR islands and their evolution as a function of applied magnetic field. At room temperature, a characteristic multidomain structure with perpendicular orientation, predicted by theory, is observed. The magnetization reversal of the islands in magnetic fields perpendicular and parallel to the substrate is dominated by strong domain wall pinning. Strong domain wall pinning in conjunction with geometrical confinement in these submicron CMR structures gives rise to reproducible multidomain structure. Further studies of the effects of the local domain state on the magnetotransport are underway.
We have incorporated a CMR electrode as a probe for the theoretically predicted half-metallicity of Fe3O4. Through the development of novel spinel insulating barrier materials grown nanometers thick, we have recently demonstrated, in CMR/barrier/ Fe3O4 junctions, junction magnetoresistances (JMRs) with more than an order of magnitude improvement as compared to previous epitaxial Fe3O4 based junctions. For example, in our ferrite-based sandwich-type tunnel junctions, a spinel structure barrier material of CoCr2O4 provides a closely lattice matched interface with the magnetite electrode and gives rise to JMRs as high as 30%. As in the case of the CMR junctions, we observe a temperature dependent JMR that we explain in terms of the Fe3O4 electrode and CoCr2O4 barrier.

Australia
The problem of exchange bias is interesting and important because in order to understand it fully, one must understand some of the most difficult issues basic to magnetism. These issues include questions of magnetic ordering in frustrated systems, exchange interactions and correlations at interfaces, and disorder and impurity effects. A theory for exchange bias moderated by partial wall formation and pinning at imperfect interfaces will be presented [1,2]. It will be shown how the magnetic and thermal stability of magnetic configurations formed on either side of the interface control the appearance of exchange bias shifts and coercivities observed in magnetization measurements.
The behaviour with temperature is particularly important. Mean field calculations show that coercive fields and the exchange bias field can display complicated behaviour with changes in temperature [3] and, taking fluctuations into account, one finds that thermal stability can be defined only relative to the rate at which equilibrium is approached [4]. The time scale on which this happens depends on the barriers to thermal activation [5-6]. The magnitude and shape of the barriers are partly determined by the exchange parameters, magnetostatic interaction energies in the ferromagnet, wall and anisotropy energies in the ferromagnet and antiferromagnet, and interface and out of plane anisotropies in the antiferromagnet.
A consequence is that measurements of the exchange bias can be sensitive to the method of measurement, with particular dependence on the time scales involved [6]. In addition to low frequency rate experiments, high frequency dynamics will also be discussed with an emphasis on possibilities for unique measures of important interface anisotropy and exchange fields [7]. Different possible experiments will be discussed which can provide measures of values determining thermal stability and governing bias properties. Results from recent high frequency measurements on model systems will be presented and discussed in terms of bias mechanisms and material inhomogeneities.
[1] J.-V. Kim, R. L. Stamps, B. V. McGrath, R. E. Camley, Phys. Rev. B 61, 8888 (2000).

A number of magnetic heterostructures exist in which pairs of ferromagnetic layers couple in anti-parallel directions. This coupling is mediated either by dipole interactions or RKKY interactions. These so-called synthetic antiferromagnets are finding increasing application in hard disk drive read heads and in ultrahigh density magnetic recording media.
We have investigated experimentally a new class of synthetic antiferromagnet involving lateral coupling between lithographically defined nanomagnets1 (Fig. 1). In this case the coupling mechanism is dipole interaction. By artificially engineering the anisotropy of each nanostructure, we can force a chain of nanomagnets to couple in an antiferromagnetic pattern. We have developed a new magnetooptical method which allows the antiferromagnetic ordering to be probed directly, despite the system having zero net moment.
We find that the first four nanomagnets in the chain couple with virtually perfect antiferromagnetic order. Thereafter, phase defects begin to arise on a statistical basis and the antiferromagnetic order parameter decays along the chain. This situation is compared to the analogous case of a chain of nanomagnets which are coupled ferromagnetically. We show that in the ferromagnetic case, a long-range soliton exists on the chain, which can mediate a high degree of ordering even down chains containing as many as 70 nanomagnets2. In the antiferromagnetic case, however, symmetry arguments show that the soliton does not exist and so the possibility of annealing phase defects from the ordering is greatly reduced.

The intriguing feature of the materials class of half-metallic ferromagnets (HMF) is metallic conductivity for one spin component and semiconducting behavior for the other, which was in most cases theoretically predicted on the basis of electron band structure calculations. An experimental struggle extended over many years and is still ongoing to convincingly verify the truly intrinsic spin-dependent electronic structure of HMFs and consequently the high spin polarization at the Fermi energy EF. The theoretically predicted 100 % spin polarization at EF of HMFs make them promising materials for magnetoelectronic and spintronic devices.

The spin-dependent electronic structure of thin epitaxial films of Fe3O4(111) (magnetite) and CrO2(100) has been investigated at room temperature by means of spin-, energy-, and energy-resolved photoemission spectroscopy. Structural properties of the eptaxial Fe3O4 and CrO2 films have been analysed by LEED, STM, and TEM. Epitaxial Fe3O4(111) films have been grown on W(110) and Al2O3(11-20) substrates and showing near the Fermi energy EF a spin polarization of –(805) % at room temperature.

The spin-resolved photoemission spectra for binding energies between 1.5 eV and EF show good agreement with spin-split band energies from density-functional calculations. Epitaxial CrO2(100) films have been deposited on TiO2(100) substrates by a chemical vapor deposition (CVD) technique. Near the Fermi energy EF an energy gap of about 2 eV was observed for spin-down electrons and a spin polarization of about (955) % was found at room temperature. After extended sputtering the spin polarization can be recovered from about +10 % up to 85 % upon annealing which would allow an ex situ CVD preparation of CrO2 films for implementing them in magnetoelectronic devices.

Generally, the performance of magnetoelectronic and spintronic devices is directly related with the spin polarization of the ferromagnets used. This fact demands a careful investigation of highly spin-polarized ferromagnetic materials which preserve an almost fully spin polarization at room temperature. In the class of HMFs Fe3O4 and CrO2 films with their high spin polarization at room temperature seem to be the most promising candidates.

Magneto-transport properties and interfaces in La0.7Sr0.3MnO3 films and heterostructuresL. Ranno

Laboratoire Louis Néel, CNRS, Grenoble, France

Ferromagnetic manganites, such as La0.7Sr0.3MnO3, are interesting models to study the magneto-transport properties of ferromagnetic materials. As well as the colossal magnetoresistive properties (CMR) which exist close to the ferromagnetic to paramagnetic transition, manganites also belong to a new class of conductors, the half metallic ferromagnets (HMF). HMF are unique in that they have a fully spin polarised conduction band, which leads to enhanced magnetoresistive properties.

Half metallicity is a very relevant property for the magnetic recording industry. Magnetoresistive properties of partially polarised materials (mainly 3d transition metals) are already used in magnetic sensors (read heads for hard disks) as well in non volatile magnetic memories (MRAM). To further improve the properties of these devices, fundamental studies of spin dependent transport mechanisms as well as the characterisation and fine tuning of new materials are required.

In the first part of this presentation, the intrinsic properties of ferromagnetic manganites, which are of interest with regard to spin-dependent transport, will be presented. Tuning of the material properties can be achieved by chemical substitution (internal chemical pressure, hole doping) or by epitaxial strain for example.

In the second part, spin-dependent transport through interfaces will be discussed with special emphasis on the spin dependent tunnel mechanism. The studied systems include granular films and epitaxial multilayers.

Low field extrinsic magnetoresistance CMR close to TC (250 K)for an epitaxial

of a granular film of La0.7Ca0.3MnO3 film of La0.7Ca0.3MnO3

deposited on MgO(001).

Influence of substrate temperature on magneto-transport properties of thin films of La0.7Sr0.3MnO3

Semiconductors in which cations are partially replaced by transition metal ions are called diluted magnetic semiconductors (DMSs). Strong exchange interactions between extended sp-carriers of the host semiconductor bands and localized d-electron of the transition metal ion cause interesting spin-dependent properties. Because strong magneto-optical activity is one of the distinguishing characters of DMS, magneto-optical spectroscopy is a powerful tool to clarify the electronic structures of DMS [1].

Since ZnO is a semiconductor with a wide band-gap (3.4eV), ZnO-based DMS would be useful for short wavelength magneto-optical applications. We investigate magneto-optical properties of Zn1-xTMxO (TM=Sc,Ti,V,Cr,Mn,Co,Ni,Cu) films to clarify the possibility of ZnO-based DMS [2]. Noticeable magneto-optical effect was not observed for Zn1-xScxO, Zn1-xTixO, Zn1-xVxO and Zn1-xCrxO. On the contrary, Zn1-xMnxO, Zn1-xCoxO, Zn1-xNixO and Zn1-xCuxO showed pronounced negative magnetic circular dichroism (MCD) peaks near 3.4eV which corresponds to the band-gap of the host ZnO semiconductor. These results show that ZnO alloyed with Mn,Co,Ni and Cu is a diluted magnetic semiconductor with strong exchange interaction between sp-band carriers and localized d-electrons. Zn1-xCoxO [3] showed the strongest magneto-optical effect. MCD and Faraday rotation of Zn1-xCoxO(x=0.012 and 0.016) at 5K are as high as 2deg/cmOe at 3.4eV, which are about two order larger than those of ZnO. Absence of strong Co2+d-d* transition near the optical band gap makes Zn1-xCoxO an useful material for short-wavelength magneto-optical applications. However, the large magneto-optical effects observed at low temperature disappear at room temperature. These ZnO-based DMSs are paramagnetic materials.

Ferromagnetic orderings in ZnO-based DMS are theoretically predicted [4,5]. Room temperature ferromagnetism of Co-doped ZnO films is also reported [6]. We discuss the origin of the observed ferromagnetism by investigating their magneto-optical spectra [7].

This work has been done in corporations with Prof. M.Kawasaki, Prof. H.Koinuma, and Dr. H.Tabata.

Interfaces and Exchange bias - A spectromicroscopy study

R.L. White and J. Stöhr
Stanford Synchrotron Radiation Laboratory, Stanford, USA - Advanced Light Source, Berkeley, USA – University Düsseldorf, Düsseldorf, Germany – Stanford University, Stanford, USA – Max Planck Institut für Mikrostrukturphysik, Halle, Germany
Today’s magnetic storage devices consist of magnetic multilayers that are magnetically coupled across their interface. While the interface itself is supposed to dominate the magnetic behavior of the entire system, the identification and characterization of its magnetic properties remains an experimental challenge. A prominent example is the loop shift (unidirectional anisotropy, exchange bias) and the coercivity increase (uniaxial anisotropy) found if a ferromagnet is coupled to an antiferromagnet. The exchange bias effect is utilized in magnetic data storage device to form a pinned magnetic reference layer. Although exchange bias was discovered over 40 years ago by Meiklejohn our understanding of its origin is still poor.
We use dichroism x-ray absorption spectromicroscopy in a photoemission electron microscope to study the magnetic coupling between antiferromagnetic NiO(001) and ferromagnetic Co across its interface. We observe large (1-20m) antiferromagnetic domains at the surface of bare NiO(001) single crystals. Upon in situ deposition of thin ferromagnetic Co layers (1.5nm) a reorientation of the antiferromagnetic axes takes place. The uniaxial anisotropy axes of the ferromagnet and the antiferromagnet are then aligned parallel domain by domain.
Spectroscopy data show that the Co deposition causes a chemical reaction and formation of an interfacial CoNiOx layer. Microscopy images reveal its polarization to be aligned parallel to the Co layer. Upon annealing both, the uniaxial anisotropy and the amount of interfacial spins increases indicating the direct link between interfacial polarization and parallel exchange coupling. An imbalance between free and pinned interfacial moments as origin of the unidirectional anisotropy will be discussed.
Our findings clearly show that a proper description of magnetic coupling in Co/NiO as well as in other AFM/FM systems needs to consider the properties of a distinct interfacial layer that can deviate significantly from the bulk properties of each material.

4Dept. of Physics, Simon Fraser Univ. Burnaby, BC, Canada V5A 1S6
Oxide-metal interfaces are of great importance in magnetoelectronics, in particular for applications of the tunneling magnetoresistance effect. In contrast to amorphous oxides which are used most often, we prepared a single crystalline system.

The geometric structure of epitaxial MgO deposited on Fe(001) in ultra high vacuum by electron evaporation was determined in detail by using surface x-ray diffraction. In contrast to common belief that MgO grows in direct contact on the Fe(001) substrate, we find  FeO interface layer between the substrate and the growing MgO-structure, which has not been considered so far. Time dependent monitoring of the (010)-antiphase crystal truncation rod intensity during deposition indicates, that the oxygen atoms are adsorbed on the Fe(001)-surface during the first stages of MgO-growth (MgO coverage <1 ML).

The analysis of the intensity distribution along four symmetry independent crystal truncation rods shows, that about 60% of the Fe(001) hollow sites are occupied by oxygen atoms leading to an outward expansion of 16% of the first Fe-interlayer distance (1.66 Å vs. 1.43 Å). The MgO-layers grow in a layer-by-layer mode, where the MgO-interlayer distances are slightly expanded as compared to the bulk.

Preliminary calculations on the basis of this structure model yield a tunneling magneto-resistance (TMR), R/R of 76% at T=0K as compared to several 1000% without the FeO-interface layer [1]. This result opens new perspectives for the understanding of the Fe/MgO/Fe(001) interfaces and the TMR effect in general.

In order to achieve spin-dependent transport in semiconductor heterostructures, diluted magnetic semiconductors (DMS) present the advantage to be more compatible with semiconductors than transition metal. In this field, the magnetoresistance of epitaxial magnetic tunnel junctions (MTJs) constitute an ideal system for the understanding of spin dependent tunnel transport [1,2] used in magnetic memory (MRAM).

We have elaborated by a low temperature molecular beam epitaxy procedure single and double barrier magnetic tunnel junctions. The two magnetic electrodes (Ga1-xMnxAs) are separated by thin AlAs (17Å) tunnel barrier (single barrier MTJ) or by a quantum well AlAs(17Å)/GaAs(50Å)/AlAs(17Å) (double barrier MTJ). Antiparallel arrangement of the ferromagnetic electrodes at low field was obtained by varying the thickness and the manganese concentration in each layer. GaAs layers have been inserted between electrodes and tunnel barrier in order to avoid the diffusion of the manganese into the tunnel barrier.

Large effects of TMR (650%) were recorded until 5T corresponding to the progressive saturation of the two magnetic electrodes. A 38% low field TMR (with the convention TMR=(Rap-R0)/R0) was observed on both single and double MTJ. This can be explained taking into account the small rate of spin-flip in the central electrode [3]. Although Tc of GaMnAs layer was reported as a maximum value of 110K this results show that it can be used as a model system to investigate spintronic effects.

To achieve ferromagnet semiconductor at room temperature, we have also started a study of the growth by laser ablation of ZnCoO layer. Preliminary results will be discussed.
[1] M. Tanaka and Y. Higo, Phys. Rev. Lett. 87, 026602 (2001).

Radiophysical Faculty, Taras Shevchenko National University of Kiev, Kiev, Ukraine

A new phenomenon of momentum relaxation reversal has been discovered experimentally and explained theoretically for dipolar spin waves propagating in yttrium-iron garnet films. It is shown that the process of momentum relaxation, caused by the scattering of a signal wave on stationary defects, can be reversed and the signal can be restituted after it left the scattering region. The reversal of momentum relaxation is achieved by frequency selective parametric amplification of a narrow band of scattered waves having low group velocities and frequencies close to the frequency of the original signal wave. This phenomenon is a classical analog of the process of stopping of a light pulse observed recently in Rb-vapor. The phenomenon of momentum relaxation reversal in yttrium-iron garnet films can be used for the development of a new type of active microwave delay lines.
Ultrafast Magnetization Dynamics in ferromagnetic and antiferromagnetic oxidesTheo Rasing

Research Institute for Materials, University of Nijmegen

Toernooiveld 1, 6525 ED Nijmegen, The Netherlands

The development of femtosecond lasers has opened a completely new area in magnetization and spin dynamics as it allows to excite/disturb and probe magnetic ordering at ultrafast timescales. We have developed time resolved linear and nonlinear magneto-optical techniques to probe the magnetization dynamics at sub picosecond time scales. The use of nonlinear optics not only allows the probing of the magnetization dynamics in 3 dimensions but also to probe the dynamics of antiferromagnetic ordering.

Recent results on ferro- and antiferromagnetic systems including oxides will be discussed.
e-mail: theoras@sci.kun.nl

FAX: 31-24-3652190

Spin injection from a ferromagnetic metal into a semiconductorH. Jaffrès

UMR 137 CNRS-Thales, Orsay, France

Achieving a spin-polarized transport into a semiconductor is one of the most important challenges in spin electronics today. What is required is a long spin lifetime in the semiconductor and an efficient spin injection of spin-polarized electrons within it. Time resolved optical experiments by Awschalom and co-workers have already shown that the spin lifetime in GaAs can be long as 100 ns at low temperature and, even at room temperature is still definitely longer than the spin lifetime in metals[1]. Injecting a large number of spin-polarized electron is a more difficult task. Two approaches have been tried. Injection from a magnetic semiconductor gives excellent results, as shown, for exemple, by the recent experiments of Fiederling et al.[2] or Ohno et al[3]. However, as long as the Curie temperature of ferromagnetic semiconductors does not exceed definitively room temperature, this approach has a limited interest for application. The second approach is based on spin-injection from ferromagnetic metals such as Co or Fe which, even at room temperature, exhibit a significantly spin-polarized conduction. However, even if one forgets technical difficulties due to the reactivity of the transition metals with most semiconductors, there remains fundamental problem of conductivity mismatch which strongly limit the spin-polarization of the injected electron, as this has been clearly shown by Schmidt et al[4].

In this presentation, we extend the calculation of Schmidt et by developing a model in which, as in the standard pictures of the perpendicular GMR, a spin dependent interface resistance is introduced between the ferromagnetic metal (F) and the semiconductor (SC). We show that the spin-polarization of the injected current can be significant when the interface resistance exceeds a threshold value related to the resistivity and spin diffusion length of the SC. For example, this can be done by introducing a tunnel barrier at the F/SC interface. We also show that a F/SC/F structure can present a significant MR if the resistances of the tunnel junctions introduced at both F/SC interfaces are in a relatively narrow range depending on the resistivity, spin diffusion length and thickness of the SC[5]. In a second part, we extend the model to the case of non-degenerate semiconductor and/or the band bending in heterostructures like Schottky diode can play a fundamental role for the efficiency of spin-injection. We must then consider a new contribution linked to the presence of a local electric field. The new point is the occurrence of asymmetrical effects for spin-injection. We detailed which are the new conditions for efficient spin-injection in ferromagnetic metal/semiconductor heterostructures.

Materials Science Center, University of Groningen, 9747AG Groningen, The Netherlands.

In many instances the properties of ultrathin films grown epitaxially and coherently on a substrate are drastically different from those of the corresponding bulk materials. Typically, coherent epitaxy is imposed by substrate overlayer interactions during the formation of the first atomic layers. In thicker layers increasing strain eventually will lead to a relaxation to the equilibrium state of the film. Kinetic barriers usually will postpone this transition until thicknesses are reached which exceed the theoretical critical value considerably. As long as the films are thin enough to be coherent with the substrate, strong modifications of the physical properties are expected. A number of examples of transition metal oxide films showing important substrate-induced changes in behaviour will be discussed.
a) Strong substrate overlayer interactions may lead to the initial stabilisation of a metastable or unstable phase. An example is the stabilisation of chromium monoxide, a non-existing compound in bulk form, if grown epitaxially on a MgO substrate. The d-Cr2+ion is of interest because it is a Jahn-Teller ion, having the same electron configuration as Mn3+, and compounds containing divalent chromium might show equally interesting electrical and magnetic properties as the manganates.
b) Coherent growth causes the films to be strained, i.e. the in-plane lattice constant is different from the out-of-plane lattice constants. The resulting symmetry lowering may cause an additional crystal field splitting of the transition metal d-levels and a corresponding change in spin- and/or orbital ordering. At present we are studying the changes in the magnetic and electronic structure of cobalt oxide on substrates having a smaller or larger lattice constant using the X-ray absorption technique.
c) Substrate-enforced strain changes the electronic band structure by increasing or decreasing the overlap between neighboring orbitals. An example under study is vanadium oxide VOx, a relatively good electronic conductor. The stoichiometry range of (stretched) films grown on MgO(001) is the same as for bulk material, but the conductivity is considerably lower. Grown on SrTiO3 (001) the (compressed) films show a much higher conductivity.
d) If the overlayer lattice is a coincidence lattice, socalled anti-phase boundaries (APB) may form in a 2D nucleation and growth mechanism. An example is the magnetite Fe3O4 spinel phase growing on top of the MgO rocksalt. The APB are structural discontinuities having a severe effect on the magneto-transport properties of the magnetite films. The effect is related to the spin-polarised nature of the transport mechanism in magnetite.
Stability of the charge order/orbital order as a function of the strain in pulsed laser deposited Ln0.5Ca0.5MnO3 thin films.Bernard Mercey

CRISMAT laboratory, CNRS UMR 6508, CAEN, France

Thin films of Pr0.5Ca0.5MnO3 and Nd0.5Ca0.5MnO3 were grown, using classical pulsed laser deposition method, on SrTiO3 and LaAlO3, [001]-oriented single crystal substrates. Taking into consideration the lattice parameters of the manganites and of the substrates, films grown onto SrTiO3 substrates present a tensile strain that depends on the thickness of the film, while films grown onto LaAlO3 substrates present a compressive strain that depends also on the thickness of the film. From x-ray diffraction and transmission electron microscopy studies, two different orientations of the film with respect to the substrate are observed, while films grown on SrTiO3 have their [010] direction perpendicular to the substrate plane, films grown on LaAlO3 have their [101] direction perpendicular to the substrate plane. Electrical transport measurements carried out in magnetic field, show that for films grown onto SrTiO3 the charge order/orbital (CO/OO) order is destabilized in magnetic fields of about 7 teslas. This destabilization renders the films highly conductive when films are normally insulating without applied magnetic field. This result proves the role of the strains induced by he substrate, since magnetic fields higher than 20 teslas must be used to destabilize this order in bulk materials. The CO/OO order of films gown onto LaAlO3 is destroy in magnetic fields of 16 teslas. Such a difference in behavior will be explained from bond length changes, resulting from the strains and from the difference between the orientation of the films with respect to the substrates.

Index

Ando, K. 11

Bobo, J.F. 12

Cowburn, R.P. 7

Hibma, T. 19

Jaffrès, H. 18

Kirschner, J. 14

Mercey, B. 20

Parkin, S.S.P 8

Ranno, L. 10

Rasing, Th. 17

Slavin, A. N. 16

Stamps, R.L. 6

Suzuki, Y. 5

This digest was printed by the Service de Reprographie, UFR des sciences.

The organizers gratefully acknowledge V. Cagan and N. Guerbignot for their help in the organisation.